The effects of supplying grains as supplementary food on nymphal development and the fecundity of adults in the rice leaf bug, Trigonotylus caelestialium (Kirkaldy), were investigated under laboratory conditions. The emergence rate of adults was 3.3% when nymphs were reared on rice seedlings alone, while the rate was 93.1–96.7% when nymphs were reared on rice seedlings with wheat grains or hulled rice. As compared with bugs reared on wheat seedlings alone, the developmental periods of nymphs reared on wheat seedlings with wheat grains or hulled rice were shorter, and the forewing length of the emerged adults was longer. The number of eggs produced by females fed on wheat seedlings with grains was 3.3–5.2-fold that of females fed on wheat seedlings alone. It was concluded that grains are very important food for the rice leaf bug, and that grain feeding raises its reproduction rate. It is suggested that the availability of seeds on the host plant affects the development and fecundity of this bug.